long-wave
radiation. Most of the terrestrialradiation
is absorbed by the water vapor in theatmosphere
and some by other gases (about 8percent
is radiated directly to outer space). Thisradiant
energy is reradiated in the atmospherehorizontally
and vertically. Horizontal flux (flowor
transport) of energy need not be considered dueto
a lack of horizontal temperature differences.The
vertical, upward or downward, flux is ofextreme
significance.

Some of this radiation is carried aloft by

convection
and turbulence. Water vapor, under.going
the condensation-precipitation-evaporationcycle
(hydrological cycle), carries the remainderinto
the atmosphere.

Atmospheric Radiation

The atmosphere reradiates to outer space most

of
the terrestrial radiation (about 43 percent) andinsolation
(about 13 percent) that it has absorbed.Some
of this reradiation is emitted earthward andis
known as COUNTERRADIATION. Thisradiation
is of great importance in the greenhouseeffect.

Heat Balance and Transfer
in
the Atmosphere

Earth does not receive equal radiation at all points
as was shown in figure 1-2-4. The east-west rotation
of Earth provides equal exposure to

sunlight
but latitude and dispersion do affect theamount
of incident radiation received. The polesreceive
far less incident radiation than theequator.
This uneven heating is called differentialinsolation.

Due to this differential insolation the tropical

atmosphere
is constantly being supplied heat andthe
temperature of the air is thus higher than inareas
poleward. Because of the expansion of warmair,
this column of air is much thicker and lighterthan
over the poles. At the poles Earth receiveslittle
insolation and the column or air is less thickand
heavier. This differential in insolation sets upa
circulation that transports warm air from theTropics
poleward aloft and cold air from the polesequatorward
on the surface. (See fig. 1-2-8.)Modifications
to this general circulation arediscussed
in detail in Unit 3.

Summary

This is the account of the TOTAL radiation.

Some
of the radiation makes several trips, beingabsorbed,
reflected, or reradiated by Earth or theatmosphere.
Insolation comes into the atmosphereand
all of it is reradiated. How many trips itmakes
while in our atmosphere does not matter.The
direct absorption of radiation by Earth andthe
atmosphere and the reradiations into spacebalance.
If the BALANCE did not exist, Earthand
its atmosphere, over a period of time, wouldsteadily
gain or lose heat.

Although radiation is considered the most

important
means of heat transfer, it is not the onlymethod.
There are others such as conduction,convection,
and advection that also play animportant
part in meteorological processes.These
will be discussed in more detail later-inthis
unit .